Thermoplasmonic Maskless Lithography on Upconverting Nanocomposites Assisted by Gold Nanostars

Abstract

Photothermal effects in plasmonic nanoparticles can be used to locally modify temperature-sensitive materials. Polylactic acid (PLA) is a thermoplastic biodegradable polymer with a glass transition temperature around 60 °C that has been popularized as a feedstock material for 3D printing. Here, we extend its use to produce thin PLA films that can be modified at the microscopic level when covered with gold nanostars (AuNSs). The heat dissipation generated when exciting the plasmon resonance of AuNSs, under exposure to 976 nm focused laser light, produces an increase in the local temperature of more than 100 °C. When the temperature surpasses the glass transition of the base PLA layer, AuNSs get attached to the polymer surface. The following dissolution of the unexposed material in acetone bath permits the precise control of the engraving process at the microscale. Furthermore, Er3+ doped upconverting nanoparticles embedded into the PLA layer can act as optical nanothermometers to probe the local temperature, simultaneously allowing the visualization of the laser spot. A computer numerical control (CNC) system was developed to drive the laser writing beam and transfer 2D patterns, opening up the thermoplasmonic maskless lithography technique. Suitable for rigid and flexible substrates coated with PLA, the methods and materials developed here were applied to produce patterned substrates for surface enhanced Raman spectroscopy and luminescent optical encoding for anticounterfeiting technologies.